1. Field of the Invention
This invention relates to polyurethane compositions having controlled adhesion to glass. More particularly, this invention relates to the use of such compositions in laminated safety glass articles, particularly motor vehicle windshields.
2. Brief Description of the Prior Art
Motor vehicle safety glass windshields have for some time been made in a trilayer configuration; namely, two plies of glass bonded together with a plastic interlayer, usually polyvinyl butyral. The trilayer windshield offers significant safety advantages over older monolithic type of windshields comprising one solid layer of glass. First, when the safety glass windshield is subjected to a fracturing impact, either from inside or outside of the motor vehicle, the plastic interlayer acts to hold the broken fragments of glass together, preventing them from flying about the inside of the car. Second, the plastic interlayer acts as a diaphragm when it is impacted and absorbs a significant amount of energy from the impacting object, thereby reducing its velocity and preventing penetration of the windshield.
Although the trilayer windshield is a significant improvement over the monolithic glass windshield, it is still, unfortunately, one of the principal sources of personal injury involving automobile accidents, being a major cause of both concussive and lacerative-type injuries.
In an effort to overcome the injury potential of trilayer windshields, there have been suggestions in the prior art to remove the inner ply of glass, thereby exposing the inner plastic ply. The inner plastic ply is generally much softer than the glass and does not present the danger of long splines of glass which can severely cut and abrade the face and head portion of an occupant impacting the windshield. Pertinent prior art on bilayer windshields appears to be the following U.S. Pat. Nos. 1,098,342 to Benedictus, 1,342,267 to Mascart, 2,047,253 to Brooks, 2,143,482 to Herrmann et al., 2,184,876 to Sherts, 2,234,829 to Neher et al., 2,454,886 to Saprio, 2,489,026 to Gilbert, 2,526,728 to Berk et al., 3,157,563 to Baum, 3,532,590 to Priddle and 3,575,790 to Fleck.
Besides bilayer windshield configurations, there have also been suggestions in the prior art to use optically clear polyurethanes as an improved safety glass interlayer for replacing polyvinyl butyral. The polyurethanes have good impact resistance over a wide range of temperatures and cause little lacerative injury and are superior to safety glass laminates employing polyvinyl butyral interlayers commercially employed at the present time. Typical patents directed to the use of polyurethanes as an interlayer material are U.S. Pat. Nos. 3,509,315 to Wismer et al. and 3,620,905 to Ahramjian and Belgium Pat. No. 785,125.
The polyurethanes are, however, pervious to moisture and additional care must be taken with bilayer configurations. With bilayers, the polyurethane is exposed on one side directly to the atmosphere with no glass sheet available as a moisture-impervious barrier. Under high humidity conditions, moisture penetrates the polyurethane from the exposed surface to the interfacial glass-polyurethane surface where it will degrade the adhesion at the interface. If humidity and temperature conditions are high enough, delamination may result.
The problem of poor adhesion under high humidity and temperature conditions can be overcome if the glass surface is primed with an organic silane or if the silane is incorporated into the polyurethane before windshield fabrication. Unfortunately, the silane usually promotes very high adhesion which minimizes the safety performance of the resultant laminated bilayer wiindshield. With very high adhesion, the bilayer behaves more as a monolithic structure in which the flexible polyurethane does not stretch and absorb energy upon impact. As a result, concussive injuries caused by an unyielding windshield can become severe. Also, very high levels of adhesion can result in reduced impact resistance of the windshield. The undesirability of high levels of glass-plastic adhesion with regards to conventional trialyer windshields has been recognized in the prior art as exemplified by U.S. Pat. No. 3,505,160 to Michaels et al. and Canadian Pat. No. 861,469 to Ammons et al.
Therefore, it would be desirable to provide an improved polyurethane composition which, when laminated to glass, provides a controlled level of adhesion to the glass and the desired impact properties. Further, it is necessary that this polyurethane composition be able to maintain its controlled level of adhesion to glass under a wide range of temperature and humidity conditions, particularly under high humidity and temperature conditions such that it can be used in bilayer safety glass applications. This invention provides such a composition and also provides resultant safety glass laminates prepared with such composition in sheet form.